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The flow through a compressor cascade with tip leakage has been studied experimentally. The cascade of GE rotor B section blades had an inlet angle of 65.1º, a stagger angle of 56.9º, and a solidity of 1.08. The final turning angle of the cascade was 11.8º. This compressor configuration was representative of the core compressor of an aircraft engine. The cascade was operated with a tip gap of 1.65%, and operated at a Reynolds number based on the chord length (0.254 m) of 388,000. Measurements were made at 8 axial locations to reveal the structure of the flow as it evolved through the cascade. Measurements were also made to reveal the effects of grid generated turbulence on this flow. The data set is unique in that not only does it give a comparison of elevated free stream turbulence effects, but also documents the developing flow through the blade row of a compressor cascade with tip leakage. Measurements were made at a total of 8 locations 0.8, 0.23 axial chords upstream and 0, 0.27, 0.48, 0.77, 0.98, and 1.26 axial chords downstream of the leading edge of the blade row for both inflow turbulence cases. The measurements revealed the formation and development of the tip leakage vortex within the passage. The tip leakage vortex becomes apparent at approximately X/ca= 0.27 and dominated much of the endwall flow. The tip leakage vortex is characterized by high streamwise velocity deficits, high vorticity and high turbulence kinetic energy levels. The result showed that between 0.77 and 0.98 axial chords downstream of the leading edge, the vortex structure and behavior changes. The effects of grid generated turbulence were also documented. The results revealed significant effects on the flow field. The results showed a 4% decrease in the blade loading and a 20% reduction in the vorticity levels within tip leakage vortex. There was also a shift in the vortex path, showing a shift close to the suction side with grid generated turbulence, indicating the strength of the vortex was decreased. Circulation calculations showed this reduction, and also indicated that the tip leakage vortex increased in size by about 30%. The results revealed that overall, the turbulence kinetic energy levels in the tip leakage vortex were increased, with the most drastic change occurring at X/ca= 0.77. / Ph. D.

The effect of global warming due to increased level of greenhouse gas emissions from coal fired thermal power plants and crisis of reliable energy resources has profoundly increased the importance of natural gas based power generation as a major alternative in the last few decades. Although gas turbine propulsion system had been primarily developed and technological advancements over the years had focused on application in civil and military aviation industry, use of gas turbine engines for land based power generation has emerged as the most promising candidate due to higher thermal efficiency, abundance of natural gas resources, development in generation of hydrogen rich synthetic fuel (Syngas) using advanced gasification technology for further improved emission levels and strict enforcement in emission regulations on installation of new coal based power plants. The fundamental thermodynamic principle behind gas turbine engines is Brayton cycle and higher thermal efficiency is achieved through maximizing the Turbine Inlet Temperature (TIT). Modern gas turbine engines operate well beyond the melting point of the turbine component materials to meet the enhanced efficiency requirements especially in the initial high pressure stages (HPT) after the combustor exit. Application of thermal barrier coatings (TBC) provides the first line of defense to the hot gas path components against direct exposure to high temperature gases. However, a major portion of the heat load to the airfoil and passage is reduced through injection of secondary air from high pressure compressor at the expense of a penalty on engine performance. External film cooling comprises a significant part of the entire convective cooling scheme. This can be achieved injecting coolant air through film holes on airfoil and endwall passages or utilizing the high pressure air required to seal the gaps and interfaces due to turbine assembly features. The major objective is to maximize heat transfer performance and film coverage on the surface with minimum coolant usage. Endwall contouring on the other hand provides an effective means of minimizing heat load on the platform through efficient control of secondary flow vortices. Complex vortices form due to the interaction between the incoming boundary layer and endwall-airfoil junction at the leading edge which entrain the hot gases towards the endwall, thus increasing surface heat transfer along its trajectory. A properly designed endwall profile can weaken the effects of secondary flow thereby improving the aerodynamic and associated heat transfer performance. This dissertation aims to investigate heat transfer characteristics of a non-axisymmetric contoured endwall design compared to a baseline planar endwall geometry in presence of three major endwall cooling features – upstream purge flow, discrete hole film cooling and mateface gap leakage under transonic operating conditions. The preliminary design objective of the contoured endwall geometry was to minimize stagnation and secondary aerodynamic losses. Upstream purge flow and mateface gap leakage is necessary to prevent ingestion to the turbine core whereas discrete hole cooling is largely necessary to provide film cooling primarily near leading edge region and mid-passage region. Different coolant to mainstream mass flow ratios (MFR) were investigated for all cooling features at design exit isentropic Mach number (0.88) and design incidence angle. The experiments were performed at Virginia Tech's quasi linear transonic blow down cascade facility. The airfoil span increases in the mainstream flow direction in order to match realistic inlet/exit airfoil surface Mach number distribution. A transient Infrared (IR) thermography technique was employed to measure the endwall surface temperature and a novel heat transfer data reduction method was developed for simultaneous calculation of heat transfer coefficient (HTC) and adiabatic cooling effectiveness (ETA), assuming a 1D semi-infinite transient conduction. An experimental study on endwall film cooling with endwall contouring at high exit Mach numbers is not available in literature. Results indicate significant benefits in heat transfer performance using the contoured endwall in presence of individual (upstream slot, discrete hole and mateface gap) and combined (upstream slot with mateface gap) cooling flow features. Major advantages of endwall contouring were observed through reduction in heat transfer coefficient and increase in coolant film coverage by weakening the effects of secondary flow and cross passage pressure differential. Net Heat Flux Reduction (NHFR) analysis was carried out combining the effect of heat transfer coefficient and film cooling effectiveness on both endwall geometries (contoured and baseline) where, the contoured endwall showed major improvement in heat load reduction near the suction side of the platform (upstream leakage only and combined upstream with mateface leakage) as well as further downstream of the film holes (discrete hole film cooling). Detailed interpretation of the heat transfer results along with near endwall flow physics has also been discussed. / Ph. D.

The turbulent flow through a marine propulsor was experimentally modeled using a large cascade configuration with six 33 cm chord flat plates spanning the entire height of the test section in the Virginia Tech Stability Wind Tunnel. Three-component hot-wire velocity measurements were obtained ahead, throughout and behind both an unstaggered and a 35º staggered cascade configuration with blade spacing and onset turbulence integral scales on the order of the chord. This provided a much needed data-set of much larger Taylor Reynolds number than previous related studies and allowed a thorough investigation of the blade-blocking effects of the cascade on the incident turbulent field. In order to generate the large scale turbulence needed for this study, a mechanically rotating "active" grid design was adopted and placed in the contraction of the wind tunnel at a streamwise location sufficient to cancel out the relatively large inherent low frequency anisotropy associated with this type of grid. The resulting turbulent flow is one of the largest Reynolds number (Reλ  1000) homogeneous near-isotropic turbulent flows ever created in a wind tunnel, and provided the opportunity to investigate Reynolds number effects on turbulence parameters, especially relating to inertial range dynamics. Key findings include 1) that the extent of local isotropy is solely determined by the turbulence generator and the size of the wind-tunnel that houses it; and 2) that the turbulence generator operating conditions affect the shape of the equilibrium range at fixed Taylor Reynolds number. The latter finding suggests that grid turbulence is not necessarily self-similar at a given Reynolds number independent of how it was generated. The experimental blade-blocking data was compared to linear cascade theory and showed good qualitative agreement, especially for wavenumbers above the region of influence of the wind tunnel and turbulence generator effects. As predicted, the turbulence is permanently modified by the presence of the cascade after which it remains invariant for a significant downstream distance outside the thin viscous regions. The obtained results support the claim that Rapid Distortion Theory (RDT) is capable of providing reasonable estimates of the flow behind the cascade even though the experimental conditions lie far outside the predicted region of validity. / Ph. D.

This study considers the ability of the audit team to configurally process information, that is, to piece together information cues held by individual team members and recognize the underlying pattern in the information. It also examines how the hierarchical structure of the audit team impacts the team's ability to process information and affects the quality of decisions made by the team. The study also considers the ability of a specific audit procedure, the fraud brainstorming session required by Statement on Auditing Standards No. 99, to overcome barriers to communication and improve team judgments in subsequent tasks. I recruited 57 dyads (114 professional auditors) from public accounting firms to complete an experimental instrument, and employed a 2x2 between-groups ANOVA, manipulating team structure (peer versus hierarchical teams) and the level of the counterfactual prime (team brainstorming session versus individual strategic prompting). I find evidence of a relationship between team structure and judgment quality, but interestingly it is in the opposite direction predicted. Research from other domains suggests differences in status within the hierarchical team may hinder communication and lead to process losses. However, I find the opposite to be true in the accounting domain. When auditors are paired in hierarchical dyads, the senior auditor assumes a leadership role, taking greater interest in the content of his/her teammate's workpapers, asking more questions, and motivating the staff auditor to volunteer a greater amount of information, which results in a higher quality judgment. Thus, this study provides initial evidence that the hierarchical nature of the audit team does not lead to the process losses documented in other domains as the assumption of a leadership role by the senior auditor allows the team to overcome any challenges inherent in the hierarchical structure. This study also considers the ability of the SAS 99 fraud brainstorming session, serving as a counterfactual prime, to lead to improved decisions later in the audit process. As predicted, the brainstorming session conducted during the planning stage of the audit increases the amount and quality of communication during the testing phase and leads to better judgments. These results are of importance for accounting firms as they determine which audit team members are required to participate in the brainstorming session. While a novice auditor may not make significant contributions to the planning decisions made during the brainstorming session, my study finds there are benefits from staff auditors participating in the brainstorming session, over and above what they are able to contribute to the session itself. Participation of staff auditors in the brainstorming session strengthens communication and enhances team-level cognition in subsequent tasks, improving the ability of the audit team to detect fraud throughout the course of the audit. These findings may be relevant for other forms of teamwork, including management teams, audit committees, and interdepartmental taskforces. / Ph. D.

The unsteady interaction of shock waves emerging from the trailing edge of modern turbine nozzle guide vanes and impinging on downstream rotor blades is modeled in a linear cascade. The Reynolds number based on blade chord and exit conditions (5*10^6) and the exit Mach number (1.2) are representative of modern engine operating conditions. The relative motion of shocks and blades is simulated by sending a shock wave along the leading edges of the linear cascade instead of moving the blades through an array of stationary shock waves. The blade geometry is a generic version of a modern high turning rotor blade with transonic exit conditions. The blade is equipped with a showerhead film cooling scheme. Heat flux, surface pressure and surface temperature are measured at six locations on the suction side of the central blade. Pressure measurements are taken with Kulite XCQ-062-50a high frequency pressure transducers. Heat flux data is obtained with Vatell HFM-7/L high speed heat flux sensors. High speed heat flux and pressure data are recorded during the time of the shock impact with and without film cooling. The data is analyzed in detail to find the relative magnitudes of the shock effect on the heat transfer coefficient and the recovery temperature or adiabatic wall temperature (in the presence of film cooling). It is shown that the variations of the heat transfer coefficient and the film effectiveness are less significant than the variations of recovery temperature. The effect of the shock is found to be similar in the cases with and without film cooling. In both cases the variation of recovery temperature induced by the shock is shown to be the main contribution to the overall unsteady heat flux. The unsteady heat flux is compared to results from different prediction models published in the literature. The best agreement of data and prediction is found for a model that assumes a constant heat transfer coefficient and a temperature difference calculated from the unsteady surface pressure assuming an isentropic compression. / Ph. D.

Renewable energy and distributed generation are getting more and more popular, including photovoltaic modules (PV), wind turbines, and fuel cells. The renewable energy sources need the power electronics interface to the utility grid because of different characteristics between the sources and the grid. No matter what renewable energy source is utilized, inverters are essential in the microgrid system. Thanks to flexible modular design, transformerless connection, extended voltage and power output, less maintenance and higher fault tolerance, the cascade inverters are good candidates for utility interface of various renewable energy sources. This dissertation proposes a new type of cascade inverters based on dual-buck topology and phase-shift control scheme. Compared to traditional cascade inverters, they have enhanced system reliability thanks to no shoot-through problems and lower switching loss with the help of using power MOSFETs. With phase-shift control, it theoretically eliminates the inherent current zero-crossing distortion of the single-unit dual-buck type inverter. In addition, phase-shift control can greatly reduce the ripple current or cut down the size of passive components by increasing the equivalent switching frequency. An asymmetrical half-cycle unipolar (AHCU) PWM technique is proposed for dual-buck full-bridge inverter. The proposed approach is to cut down the switching loss of power MOSFETs by half. At the same time, this AHCU PWM leads to current ripple reduction, and thus reducing ripple-related loss in filter components. Therefore, the proposed PWM strategy results in significant efficiency improvement. Additionally, the AHCU PWM also compensates for the zero-crossing distortion problem of dual-buck full-bridge inverter. Several PWM techniques are analyzed and compared, including bipolar PWM, unipolar PWM and phase-shifted PWM, when applied to the proposed cascade dual-buck full-bridge inverter. It has been found out that a PWM combination technique with the use of two out of the three PWMs leads to better performance in terms of less output current ripple and harmonics, no zero-crossing distortion, and higher efficiency. A grid-tie control system is proposed for cascade dual-buck inverter with both active and reactive power flow capability in a wide range under two types of renewable energy and distributed generation sources. Fuel cell power conditioning system (PCS) is Type I system with active power command generated by balance of plant (BOP) of each unit; and photovoltaic or wind PCS is Type II system with active power harvested by each front-end unit through maximum power point tracking (MPPT). Reactive power command is generated by distributed generation (DG) control site for both systems. Selective harmonic proportional resonant (PR) controller and admittance compensation controller are first introduced to cascade inverter grid-tie control to achieve better steady-state and dynamic performances. / Ph. D.

Gas turbine companies are always focused on reducing capital costs and increasing overall efficiency. There are numerous advantages in reducing the number of airfoils per stage in the turbine section. While increased airfoil loading offers great advantages like low cost and weight, they also result in increased aerodynamic losses and associated issues. The strength of secondary flows is influenced by the upstream boundary layer thickness as well as the overall flow turning angle through the blade row. Secondary flows result in stagnation pressure loss which accounts for a considerable portion of the total stagnation pressure loss occurring in a turbine passage. A turbine designer strives to minimize these aerodynamic losses through design changes and geometrical effects. Performance of airfoils with varying loading levels and turning angles at transonic flow conditions are investigated in this study. The pressure difference between the pressure side and suction side of an airfoil gives an indication of the loading level of that airfoil. Secondary loss generation and the 3D flow near the endwalls of turbine blades are studied in detail. Detailed aerodynamic loss measurements, both in the pitchwise as well as spanwise directions, are conducted at 0.1 axial chord and 1.0 axial chord locations downstream of the trailing edge. Static pressure measurements on the airfoil surface and endwall pressure measurements were carried out in addition to downstream loss measurements. The application of endwall contouring to reduce secondary losses is investigated to try and understand when contouring can be beneficial. A detailed study was conducted on the effectiveness of endwall contouring on two different blades with varying airfoil spacing. Heat transfer experiments on the endwall were also conducted to determine the effect of endwall contouring on surface heat transfer distributions. Heat transfer behavior has significant effect on the cooling flow needs and associated aerodynamic problems of coolant-mainstream mixing. One of the primary objectives of this study is to provide data under transonic conditions that can be used to confirm/refine loss predictions for the effect of various Mach numbers and gas turning. The cascade exit Mach numbers were varied within a range from 0.6 to 1.1. A published experimental study on the effect of end wall contouring on such high turning blades at high exit Mach numbers is not available in open literature. Hence, the need to understand the parametric effects of endwall contouring on aerodynamic and heat transfer performance under these conditions. / Ph. D.

Airfoil endwall contouring has been widely studied during the past two decades for the reduction of secondary losses in turbine passages. Although many endwall contouring methods have been suggested by researchers, an analytical tool based on the passage design parameters is still not available for designers. Hence, the best endwall contour shape is usually decided through an optimization study. Moreover, a general guideline for the endwall shape variation can be extrapolated from the existing literature. It has not been validated whether the optimum endwall shape for one passage can be fitted to other similar passage geometry to achieve, least of all a non-optimum but a definite, reduction in losses. Most published studies were conducted at low exit Mach numbers and only recently some studies on the effect of endwall contouring on aerodynamics performance of a turbine passage at high exit Mach numbers have been published. There is, however, no study available in the open literature for a very high turning blade with a transonic design exit Mach number and the effect of endwall contouring on the heat transfer performance of a turbine passage. During the present study, a robust, aerodynamic performance based numerical optimization methodology for turbine endwall contouring has been developed. The methodology is also adaptable to a range of geometry optimization problems in turbomachinery. It is also possible to use the same methodology for multi-objective aero-thermal optimization. The methodology was applied to a high turning transonic turbine blade passage to achieve a geometry based on minimum total pressure loss criterion. The geometry was then compared with two other endwall geometries. The first geometry is based on minimum secondary kinetic energy value instead of minimum total pressure loss criterion. The second geometry is based on a curve combination based geometry generation method found in the literature. A normalized contoured surface topology was extracted from a previous study that has similar blade design parameters. This surface was then fitted to the turbine passage under study in order to investigate the effect of such trend based surface fitting. Aerodynamic response of these geometries has been compared in detail with the baseline case without any endwall contouring. A new non-contoured baseline design and two contoured endwall designs were provided by Siemens Energy, Inc. The pitch length for these designs is about 25% higher than the turbine passage used for the endwall optimization study. The aerodynamic performance of these endwalls was studied through numerical simulations. Heat transfer performance of these endwall geometries was experimentally investigated in the transonic turbine cascade facility at Virginia Tech. One of the contoured geometries was based on optimum aerodynamic loss reduction criterion while the other was based on optimum heat transfer performance criterion. All the three geometries were experimentally tested at design and off-design Mach number conditions. The study revealed that endwall contouring results in significant performance benefit from the heat transfer performance point of view. / Ph. D.